In certain video disc systems, video information is recorded by means of geometric variations in the bottom of a smooth spiral groove on the surface of a disc record. The disc record surface includes a coating of conductive material which is preferably covered with a thin deposit of dielectric material. A signal pickup, supported by a pickup arm, engages the spiral groove and includes a conductive electrode which establishes a capacitance with the conductive coating and the dielectric deposit of the disc record. If the disc record is disposed on a turntable of the player in a playback position, the conductive coating is located remote from the turntable. When the disc record is rotated, the signal pickup/disc record capacitance varies in response to the geometric variations in the bottom of the spiral groove passing beneath. The capacitance variations are converted to electrical variations by a suitable signal processing circuitry coupled to the signal pickup electrode. The output signal of the signal processing circuitry may be fed to a conventional television receiver for audio/visual presentation. The other end of the pickup arm is releasably secured by a coupler to a support member of a supporting structure of the playback system. A system of the aforementioned type is described in detail in the U.S. Pat. No. 3,842,194, issued to John Kaufmann Clemens, on Oct. 15, 1974, and entitled "INFORMATION RECORDS AND RECORDING/PLAYBACK SYSTEMS THEREFOR".
Illustratively, (as described, for example, in the copending U.S. Pat. application, Ser. No. 295,854, filed Oct. 10, 1972, for S. E. Hilliker, entitled "VIDEO DISC TRANSMISSION LINE AND STYLUS RF RETURN systems", and now U.S. Pat. No. 3,872,265), the capacitive variations between the conductive coating on the disc record and the conductive electrode of the signal pickup are a part of a tuned circuit, wherein the resonant frequency of the tuned circuit varies as the capacitance between the signal pickup and the disc record changes. A flexibly mounted conductive member (e.g., pickup arm) is provided, with the free end coupled to the conductive electrode of the signal pickup, and the pivoted end movably connected to a hollow box-like support housing also made of conductive material. The conductive member mechanically supports the signal pickup, and also forms a transmission line with the surrounding conductive support housing. The conductive coating side of the variable disc record/signal pickup capacitance is returned to the conductive housing via a capacitance established between (1) the disc record conductive coating and a portion of the conductive housing overlying the disc record conductive coating, and/or (2 ) via a capacitance established between the disc record conductive coating and a conductive portion provided in the turntable which is, in turn, connected to the conductive support housing during playback. A tuned circuit is established which includes (1) reactance of the transmission line, (2) the capacitance between the signal pickup electrode and the disc record conductive coating, and (3) the capacitance between the conductive housing and the disc record conductive coating, and/or the capacitance between the disc record conductive coating and the turntable conductive portion. The tuned circuit is energized by a fixed, high frequency (for example, 915 MHz) oscillator. The response of the tuned circuit to the energization signal from the oscillator changes as a function of the prerecorded information on the disc record. This provides output signals, representative of prerecorded information on the disc record, which are subsequently fed to a suitable signal processing circuit.
In order to obtain adequate bandwidth for monochrome and color display in video disc systems, the disc record is rotated at relatively high playback speeds (e.g., in some video disc systems up to 1,500 rpm) as compared with low playback speeds (e.g., 331/3 rpm) associated with conventional audio disc systems. The use of high speed of disc record rotation entails a rapid rate of intimate relative motion between the disc record groove surface (e.g., polystyrene coating) and the conforming signal pickup tip (e.g., finely lapped sapphire or glass base) as the signal pickup rides in the disc record groove during playback, which results in charge accumulation on the disc record surface of several hundred volts (e.g., up to 2,000 volts). Moreover, it has been also found that as the disc record having two conductive coatings (e.g., the disc record having prerecorded information on both sides) separated by the dielectric substrate is pulled out from the disc record storage sleeve, electrical charge builds up on the dielectric deposits overlying the conductive coatings. The electrical charge on the dielectric deposits induces opposite charge on the adjacent disc record conductive coatings.
The build-up of electrical charge on the disc record conductive coating is undesirable for several reasons. First, the charge build-up is a safety hazard. It may cause arcing with the surrounding metal structure with a possibility of initiating an unfriendly fire. Also, it may give an electric shock to humans upon an accidental contact with the charged disc record while handling the player.
Second, in the playback systems of the aforementioned type, the following considerations may be encountered. One, groove-engaging and low mass requirements of such a typical signal pickup apparatus utilized in the above-mentioned systems results in a pickup arm unit which is extremely fragile (e.g., signal pickup width 2 microns, and signal pickup depth 5 microns). Two, the dimension of the electrode of the signal pickup along the spiral groove of the disc record must be commensurate with the resolution requirements of high frequency signals (e.g., 4.3 to 6.3 MHz) recorded in the bottom of the disc record groove (illustratively, signal pickup electrode dimension along the groove of 0.2 microns). Therefore, electrical charge on the disc record may discharge through the delicate signal pickup electrode causing permanent damage to the groove-engaging signal pickup.
As indicated above, the A-C return path for the conductive coating side of the signal varying disc record/signal pickup capacitance must be provided. It may also be advantageous to provide a D-C return path for the conductive coating side of the signal varying disc record/signal pickup capacitance in order to remove electrical charge accumulated on the disc record conductive coating which may deleteriously affect the performance of the video disc playback system. Therefore, it is desirable to ground the disc record conductive coating to a reference potential (e.g., ground potential) of the playback system.
Further, in order to insure that the disc record/signal pickup capacitance variations produce variations in the resonant frequency of the tuned circuit of adequate magnitude, it is desirable to make the series capacitance established between the disc record conductive coating and the conductive housing overlying the disc record conductive coating as large as possible. The capacitance of a capacitor is (1) directly proportional to the area of the capacitor plates, (2) inversely proportional to the separation between the capacitor plates, and (3) directly proportional to the dielectric constant of the material separating the capacitor plates. For a given area of the capacitor plates and a specified dielectric material separating the capacitor plates, the capacitance of the capacitor may be made large by locating the conductive housing as close as possible to the disc record conductive coating during playback. Thus, if the conductive housing is spaced close to the disc record conductive coating (e.g., 0.050 to 0.150 inches), addition of a second disc record (e.g., 0.075 inches thick) on top of a first disc record may cause irreparable damage to the second disc record, and/or to the record-engaging signal pickup. Thus, it is desirable to provide an apparatus for inhibiting the introduction of a second disc record into operable position on the turntable in the presence of a first disc record disposed in the operable position on the turntable.